Over the past twenty years, there has been considerable interest in developing zeolite-based membranes for gas or liquid separations, reactor/separators, and sensor/detector applications. The potential cost savings in reduced energy consumption of low temperature gas separations by physical means rather than using cryogenic methods has motivated this research in developing microporous membranes. Polymer films, carbon molecular Sieves, ceramic membranes, and zeolite membranes have all attracted attention for various applications. Zeolites in particular, due to their inherently regular pore dimensions on the molecular scale and high thermal stability, have been the subject of numerous studies.
The most common method of synthesizing zeolite-based membranes has been to place a host substrate, for example a macroporous support surface, into a zeolite reagent solution, such as solutions conventionally used for hydrothermal zeolite synthesis, in some particular orientation, and simply allow the zeolite crystals to form and grow on the substrate surface. Variations of this method include applying seeded gel mixtures or gel solutions on amorphous substrates to form polycrystalline zeolite aggregates on the host surface. The primary requirement of the host surface in these methods is that the surface remain stable in a hydrothermal environment with pH of up to about 13.5 and temperatures up to about 180.degree. C. Prolonged exposures under such conditions require that host substrates resist dissolution, except in the case of clay supports where the substrate provides some of the material for zeolite formation [see Engelen, C. W. R. and van Leeuwn, W. F., "Membrane for Separating Off Small Molecules and Method for the Production Thereof," International Patent (Mar. 27, 1992)].
Numerous studies have disclosed such hydrothermal synthesis methods [see Tsikoyiannis, J. G. and Haag, W. O., ZEOLITES 12 (1992) 126-130; Sano, T., Kiyozumi, Y., Mizukami, F., Takaya, H., Mouri, T., and Watanabe, M., ZEOLITES 12 (1992) 131-134; Jansen, J. C., Kashchiev, D., and Erdem-Senatalar, A., Stud. Surf. Sci & Catal., No. 85, Jansen, J. C., Stocker, M., Karge, H. G., and Weitkamp, J., eds. (1994) 215-250; Van Bekkum, H., Geus, E. R., and Kouwenhoven, H. W., Stud. Surf. Sci. & Catal., No. 85, Jansen, J. C., Stocker, M., Karge, H. G., and Weitkamp, J., eds. (1994) 509-542; Yan, Y., Davis, M. E., and Gavalas, G. R., Ind. Eng. Chem. Res. 34 (1995) 1652-1661; Lavallo, M. C., Gouzinis, A., and Tsapatsis, M., AIChE J. 44 (1998) 1903-1913; Xomeritakis, G., Gouzinia, A., Nair, S., Okubo, T., He, M., Overney, R. M., and Tsapatsis, M., Chem. Eng. Sci. 54 (1999) 3521-3531] In one variation of these synthesis methods, MFI seed solutions have been utilized to minimize the depth of layers of crystals [see Lavallo, M. C., Gouzinis, A., and Tsapatsis, M., AIChE J. 44 (1998) 1903-1913; Xomeritakis, G., Gouzinia, A., Nair, S., Okubo, T., He, M., Overney, R. M., and Tsapatsis, M., Chem. Eng Sci. 54 (1999) 3521-3531; Albers, E. W. and Grant, C. E., U.S. Pat. No. 3,730,910].
Typically, such approaches result in the formation of randomly oriented polycrystalline films on the host surfaces which have well defined grain boundaries between the individual crystals. Grain boundaries are "sealed" to some extent by allowing the polycrystalline mass to grow in the third dimension, that is to make the grain boundaries thick. More specifically, since randomly positioned crystals on a support surface have no crystallographic orientation, very weak bonds form between crystals, if at all, and the defects are "sealed" by allowing the depth of the grain boundaries to be sufficiently large.
Problems associated with these approaches to zeolite membrane synthesis have been documented in recent studies [see Saracco, G., Neomagus, H. W. J. P., Versteeg, G. F., and van Swaaij, W. P. M., Chem. Eng. Sci. 54 (1999) 1997-2017; Szostak, R., "Molecular Sieves: Principles of Synthesis and Identification," 2.sup.nd edition (1998) Blackie Acad. & Prof., London]. The deficiencies of these methods include formation of numerous defects (leaks) along grain boundaries, very weak mechanical strength along the grain boundaries, low fluxes of gases due to the excessive depth of the zeolite layer, poor selectivity due to the random orientation of pore structures normal to the flux, and poor adhesion between the substrate and zeolites. Attempts to overcome these limitations by reducing the depth of the zeolite layer to increase the flux through the "gate-keeper" layer results in decreased mechanical strength due to the lack of bonding between randomly oriented crystallites [see Gonthier, S. and Thompson, R. W., Stud. Surf. Sci. & Catal., No. 85, Jansen, J. C., Stocker, M., Karge, H. G., and Weitkamp, J., eds. (1994) 43-73.]. The resultant materials that are typically produced by such methods are undesirable for commercial applications since vibrations resulting from pumps, compressors, and high fluid flow rates in actual industrial settings are likely to fracture the weak zeolite layers resulting in premature membrane failure or deterioration.
In a recent study, ZSM-5 crystals have apparently been incorporated into conventional polymer films and selective separations have been achieved [see Duval, J. -M., Kemperman, A. J. B., Folkers, B., Mulder, M. H. V., and Desgraddchamps, G., J. Appl. Polym. Sci. 54 (1994)409-418]. U.S. Pat. No. 4, 973,606 teaches insertion of zeolites into polymers, such as thermoplastic elastomers or duromers, for producing membranes with controllable selectivity for material separation. U.S. Pat. No. 5,069,794 teaches application of zeolite coatings to different substrates to act as thin membranes. However, these approaches appear to be somewhat limited since they apparently do not enhance the separation capabilities of the polymer matrix component of the membrane film. Furthermore, films produced by this method are apparently fragile, and difficult to manipulate, suggesting that such films would be impractical for industrial applications. These polymer films have additional limitations due to their low thermal stability since they either decompose or melt at relatively moderate temperatures. Decomposition of such materials typically produces undesirable porosity and carbonaceous residues.
Incorporating molecular sieve zeolites into silica films has been previously reported by Bein, et al. [see Bein, T., Brown, K., Enzel, P., and Brinker, C. J., Mat. Res. Soc. Symp. Proc., Vol 121 (1988) 761-766]. However, this approach utilizes tetraethylorthosilicate (TEOS) as the sole source of silica. Such films have exhibited impermeability to nitrogen as the use of pure TEOS apparently limits control of the membrane matrix porosity. The resultant film matrix has very low porosity and small pore dimensions and there appears to me little flexibility in tailoring porosity with such membrane matrix materials. This matrix limitation apparently requires that the maximum thickness of the membrane matrix matches the dimension of the zeolite crystals such that zeolite crystal faces protrude from both sides of the membrane matrix. Such a requirement poses a significant limitation where incorporating high surface area, sub-micron zeolite crystals is desirable.
One limitation of all of the above methods is that forming a zeolite film coating with such approaches typically requires high-temperature processing that degrades the organic component. In membrane applications, materials formed by high temperature methods are generally undesirable due to their impermeability to gas. In addition, with such methods it is difficult to control the porosity of the conventional organic polymer matrices in which the zeolites are layered. Another significant limitation is the poor adhesion of composite membrane layers on various surfaces and poor bonding between the matrix and the zeolite.
Thus, it is desirable to provide a polymer containing zeolites with optimal permanent bonding between zeolite and matrix that can be obtained at low temperatures and that simultaneously permit good adhesion to substrates and controlled membrane porosity. If a hybrid inorganic/organic polymer synthesis method can be successfully developed for applications in fabricating zeolite polymer membranes, it would be possible to overcome many of the limitations of these prior art membranes.